Many of today's computers, cellular telephones, and other such devices rely on wireless communication to exchange information with other devices, such as to surf the Internet, send and receive email, and communicate audio and video. Many such devices also include GPS, useful in that it enables a person to determine where they are, overlay their location on a map, and get directions. GPS signals are often also used by other applications, such as a restaurant review application that shows reviews of restaurants near a user, or a shopping application that shows stores near a user that have a particular item for sale.
But, GPS signals are difficult to receive in some cities with tall buildings or other interference, and GPS is often left out of lower cost devices. Some devices therefore use cellular phone towers or other radio signals having known location to use as a reference, and can determine approximate position of a mobile radio device by estimating the distance from the mobile wireless device to multiple cellular towers. This can be performed such as by using chirp signals and measuring the time of flight between RF devices, but accuracy is dependent in part on synchronization between the two devices.
Chirp signaling may also be used in communication systems. In a chirp-spread system, when the offset of the carrier frequencies between the transmitter and the receiver becomes sufficiently large the frequency offset will appear as a time offset at the receiver. Chirp-modulated communication systems which have relatively large frequency spacing between samples (i.e., Fs/N) will not experience this time offset shift assuming the relative oscillator inaccuracy is smaller than the spacing. In other chirp-modulated communication systems, the time and frequency offset may not be corrected, and, in these cases, the symbol timing at the receiver may be misaligned with the received data. This produces a non-optimal partial correlation or intersymbol interference (ISI) that degrades the sensitivity of the receiver.
Additionally, if the receiver has no knowledge of the frequency offset it tracks the signal based upon the demodulated symbols in order to maintain synchronization. However, if the frequency offset is known at the receiver, it can use a more robust means of tracking the signal. If the symbol clock and the carrier frequency are derived from the same oscillator at the transmitter and receiver, the frequency offset is proportional to the time drift. Thus, the frequency offset information can be used to track the incoming signal. This method of tracking is much more robust in high interference or low signal to noise environments. A third reason for correcting the time and frequency offset is to ensure a common receive phase among chirp offsets when employing orthogonal chirp modulation to convey data.
For these and other reasons, it is desired to synchronize two devices using chirp signals for RF communication.